1982 G1 rebuilt, ran once, now only backfires

[JENNAKELLY5]

Thanks for the feedback cart around and slonomo.
I got home from work tonight and the first thing i did was grab a spark plug spanner and a torch. Looking into the cylinder i can see an "EX" printed on the piston and it's facing towards the exhaust, so I think I've accidentally got that right. (Assuming EX means exhaust).

[cart around]

Good news about your piston.

Now, how about your rings? http://www.golfcarnews.com/gcn/?m=200805

[JENNAKELLY5]

It's too late over here to get on the tools now, but I don't think I would have been lucky enough to get that right. I'll check that first thing tomorrow and I'm actually hoping I have done it wrong and that is my only problem, fingers crossed.
Thanks again for the info cart around, you're a legend!

[smallblock450sl]

Did you change the crank seals on the rebuild? Did you "mark" the timing plate so it would go back into the same location?

[cart around]

Don't thank me yet. I'm only throwing guesses out there, and can be completely wrong.

[Volt_Ampere]

It does sound like the timing is way off.

[cart around]

Let's think this through for a minute.

We have a fresh rebuild that cranked for 15 seconds until the choke was pulled, and then drove across the yard. That sounds like the right amount of time to get the fuel flowing, and for everything to get primed.

The problem didn't start until he stopped, and tried to run it in reverse. Just starting up, and running steers me away from a major timing flaw unless something moved.

It could be still be an ignition problem, fuel flow problem, air leak, or low compression.
I wouldn't mind seeing a compression test. I would also like to see a shot of ether to see if it's a fuel problem.

My ideas could be completely wrong. I will be curious to see what fixes this one.

[sho305]

Make sure the clutch taper is really clean and also inside the clutch. The best thing to do is put a little valve lap compound on it and press and turn the clutch on the crank for a few minutes (also a good trick on flywheels). This will clean and lap the tapers together for a better fit. Clean all the stuff off really good and put it back together. Gokart racers lap flywheels and use no key and they stay long as they don't backfire, so they can time them differently that way. Don't loctite the bolt later you might never get it out.

As I recall the timing plate can't move very much, but could still come loose maybe? Always check for wiring problems that is a legendary but cheap thing to fix on old yamaha. Always check the ground wiring system (all grounds are wired unlike a car). Get a spark checker they are cheap, and you can see exactly what spark you have going on. Check compression you could have a gasket issue or ring problem. Pop the exhaust lose and try to start it, common for G1 muffler to plug up. Double check piston like said above, check crank seals just in case as leak in bottom end will cause this. Running then dying would lead me to something that was working but failed (gasket blew?), something got hot enough to fail (electrical), or pre existing problem showed up like muffler plugged, bad pulse line or pump, etc. Shoot some fuel in the plug hole to check for fuel issue, see if it runs.

[JENNAKELLY5]

Ok, so over the weekend I pulled the engine out again to check the rings. The rings are fine, but I did notice that my woodruff key is busted. There was enough gunk in there that the fan and CDI(maybe?) would still spin occasionally, but not consistently. I'm trying to find out if I can buy one in Australia or alternatively machine one from some scrap. Worst case I'll get one from America, but it will just mean a few weeks of waiting.
Also with the timing plate, I marked it's position but this could have been incorrect from a previous owner. Is there a general area where they should be or does it vary a lot? The screws are pretty close to the middle of the slots.

[TheGDFP]

Key seems to fit a few Yamaha models.
http://www.partzilla.com/parts/detai...-16053-00.html